The present invention relates to an ultrasound imaging system, and more particularly, to a three-dimensional ultrasound imaging system for performing receive-focusing at voxels corresponding to pixels of a display device.
Ultrasound imaging systems are widely used in the medical diagnostic field for their ability to obtain the image of an object non-invasively, i.e., by transmitting ultrasound signals to the object and processing their reflection. conventional, three-dimensional (3D) ultrasound imaging systems have an array of ultrasound transducers or probes for generating ultrasound pulses and receiving echo signals of the ultrasound pulses reflected off an object. Ultrasound pulses from the array of ultrasound transducers are transmit-focused to a desired point by controlling the timing of the ultrasound pulse generation at each of the transducers. Specifically, the timing control of the ultrasound pulse generation compensates for propagation delays due to the different distances between respective transducers and particular points on a scan line. By sequentially delaying the generation of ultrasound pulses from the transducers, all the ultrasound pulses are simultaneously focused to a particular point. Simultaneous reception of the reflected ultrasound pulses by the array of transducers is also made possible by sequentially adjusting the receive-timings of respective transducersxe2x80x94the greater the distance from a respective transducer to a particular point, the more receive delay provided to that transducer.
In order to obtain an accurate 3D image of an object, transmit-focusing to multiple points on the target object is needed. But after the transmission of ultrasound pulses to the target point, transmission to another target point must wait until all the reflected ultrasound pulses are received, including the one reflected from the farthest point. Increasing the number of transmit focal points has the drawback of increasing the amount of time required to obtain an 3D image, thus reducing the frame rate.
Where the transmission focuses on a single point, the frame rate is determined by the following equation:                               1          FR                =                                            2              ⁢                              xe2x80x83                            ⁢              D                        c                    xc3x97          N                                    Eq.   (1).            
FR is the frame rate; D is the depth of the scan; c is the velocity of ultrasound transmission in the medium; and N is the number of scan lines. As seen in Eq. 1, the frame rate is inversely proportional to the number of scan lines. Thus, there is a trade-off between frame rate and the number of scan lines.
One conventional solution is the sequential application of a radial scan pattern over the entire diagnostic area along N number of the scan lines to predetermined points. In addition to a radial scan pattern, a parallel scan line pattern has also been widely used. With these scanning methods, i.e., dynamic receive-focusing methods, the receive-focusing is achieved only on points along the scan lines, limiting collection of data of an object to these points of the scan lines.
Referring to FIG. 1, 3D ultrasound imaging system 100 includes transducer array 102, beamformer 104, envelope detector 106, log compensator 108, digital scan converter 110, image former 112, and display device 114. Envelope detector 106 and log compensator 108 constitute echo signal processing unit 116. Array 102 sequentially transmits ultrasound pulses to be focused on target points on the scan lines of an object. After transmitting the ultrasound pulses to one of the scan lines, the respective transducers receive echoes reflected from the target points. Beamformer 104 focuses the received echo from a target point on the scan lines for storage in the form of radio frequency (RF) data. Beamformer 104 repeats this receive-focusing for each of the target points on every scan line, to acquire data about the shape of the object. The data acquired by beamformer 104 is processed through envelope detector 106, log compensator 108, and digital scan converter 110, to thereby become a 3D data set used in obtaining desired 3D images.
Display device 114 generally has pixels arranged in a matrix on its screen and each pixel should be provided with display data to form a 3D image. Digital scan converter 110 first stores data which is receive-focused and the converts the data to a horizontal raster line display format used in most display devices. The converted data is the 3D data set. The 3D data set of the object, acquired by using the dynamic receive-focusing scheme, is limited to the focused target points on the scan lines. These focused target points do not necessarily coincide with actual pixel points on the display device (these actual pixel points corresponding to the pixel locations on the display device will simply be referred to as xe2x80x9cpixel pointsxe2x80x9d hereafter). Thus, digital scan converter 110 has to perform 3D (Rxe2x88x92xcex8) interpolation, as is well known in the art, on the 3D data set to provide display data for all the pixels of the display device. For example, in the case of a radial scan pattern, because the distance between adjacent scan lines become greater as you get farther away from the transducers, the number of pixel points is greater where a 3D data set is not acquired directly from the receive-focused data. Using the 3D data set from digital scan converter 110, image former 112 forms a 3D image to display on display device 114. Digital scan converter 110 must also perform interpolation in the case of a parallel scan pattern.
Thus, conventional 3D ultrasound imaging systems first obtain a 3D data set after forming a 2D image, by using display data acquired in one frame, and then form a 3D image based on that 3D data set. Consequently, the quality of the 3D image is dependent on the 2D image. Because conventional systems must perform this digital scan conversion to obtain 3D data sets, some distortion is introduced into the 3D image.
It is, therefore, an object of the present invention to provide a three-dimensional (3D) ultrasound imaging system for forming a 3D image comprising: a display device; at least one transducer, for transmitting ultrasound signals toward an object and receiving echo signals from a voxel corresponding to a pixel on the display device, wherein the voxel is on a scanning region of the object; signal storing means for storing signals from at least one of the transducers; signal processing means for processing the stored signals to obtain 3D data sets with respect to the voxel; and image forming means for forming a 3D image based on the 3D data sets.